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Re: [OM] OM Shutter Questions

Subject: Re: [OM] OM Shutter Questions
From: Chris Crawford <chris@xxxxxxxxxxxxxxxxxxxxxx>
Date: Wed, 14 May 2014 01:07:06 -0400
Chuck,

You're right about the OTF metering on the OM bodies. If the exposure is
too short for the whole frame to be exposed at once, the metering is done
right before the exposure, off the shutter curtain. That's why OM bodies
with autoexposure have the funny pattern of black and white dots all over
the first curtain. Partially this gives the curtain the right reflectance
for accurate metering, and partly it defines the metering pattern.

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On 5/13/14 7:59 PM, "Chuck Norcutt" <chucknorcutt@xxxxxxxxxxxxxxxx> wrote:

>Yes, I think your OM thinking is rusty.  OM shutters (except for the
>oddball OM-2000) run their cloth shutter curtains from side to side and
>not top to bottom as with metal curtain shutters like the OM-2000.
>
>I've never thought much about curtain travel time but think it must be
>the same as the max synch speed.  Consider the shutter curtain actions
>at OM max sync speed of 1/60 second.  In order for the full film frame
>to be exposed to very high speed electronic flash the first shutter
>curtain must be fully open and the second curtain must not yet have
>started to close.  That means the first curtain must have gotten fully
>open in 1/60 second and the second curtain is now immediately starting
>to close.  If it travels in 1/60 second also the entire frame will have
>been exposed for 1/60 second but there was a very brief time when the
>entire frame was exposed for flash.
>
>Vertical metal blade shutters have faster sync times because they have a
>shorter distance to travel.
>
>I dunno but I think TTL/OTF exposure above sync speed must be determined
>entirely from the shutter curtain before it opens and not OTF.  Since
>shutter speeds above sync speed are formed by a traveling slit the
>timing of the release of the second curtain must be known at the time
>the first curtain starts moving.  Otherwise, exposure determination
>couldn't be done across the entire frame and one couldn't change
>exposure once the first curtain started traveling... a varying slit
>width on a single exposure would be unwelcome.
>
>Chuck Norcutt
>
>
>
>On 5/13/2014 7:10 PM, Mike Gordon via olympus wrote:
>> raises the question of how fast the OM shutter
>> is above the 1/60 sync speed??? (i.e time to totally expose the film
>> from top to bottom)  It must be very fast after the magnet  signaled to
>> release the 2nd curtain is released as never recall "rolling shutter"
>> artefacts.
>> I should know this, but must be rusty thinking about OM.   A related
>> question is the TTL/OTF metering.  Above sync speed perhaps is hybrid of
>> OTF and off the curtain until exposure quench point reached for 18%??
>> Below sync speed must be 100% OTF.
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